1. Field of the Invention
This invention relates to a layout and method for longwall mining of thick underground mineral seams and more particularly to a method for increasing the recovery from such an operation.
2. Description of the Prior Art
For the purposes of this application, a "thick seam" is defined as a seam too thick for maximum face productivity using existing mining systems. According to T. S. Cochrane in a paper entitled "Underground Mining of Thick Coal Seams" presented at the 74th Annual General Meeting of the CIM, Ottawa, Canada, productivity begins to fall off at between 61/2 feet and 13 feed depending upon whether integrated mechanized systems or labor-intensive systems are used.
In the mining of thick underground mineral seams, for example of coal, a mining area may be initially blocked out by submain entries and bleeder entries between which parallel sets of so-called panel entries may be driven. Adjacent sets of panel entries are spaced apart so as to define a longwall panel therebetween which may be mined by known techniques, usually in retreat. If the longwall face is developed at the bottom of the coal seam and mined under the protection of advancing roof supports, additional coal is recovered by caving of overlying strata on the gob side behind such advancing roof structures.
At least two problems exist with existing layouts and methods of the character described. In the first place, if the submains are driven into the bottom of the seam, it may be difficult to protect these entries against roof fall because of the pressure of the overburden and the character of the roof formations. This additionally requires tunneling from the surface to a lower depth. Secondly, once the longwall panels have been mined in retreat for a percentage of the distance back toward submains, the operation usually ceases. No attempt is made to recover the coal lying above the submains, and this may represent a considerable loss.